mxser: access info->MCR under info->slock

info->MCR is protected by info->slock all over the code. Extend the lock
in mxser_tiocmget around the info->MCR fetch too.

It likely doesn't change anything, but it's always good to be consistent.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-64-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>